In EP 0 619 801 B1 it is stated as a problem that the carbon in composites having a ceramic matrix and reinforced with carbon fibers, for example C/SiC materials (carbon-silicon carbide), tends to oxidize, especially in applications at a relatively high temperature. This behavior is a particular disadvantage if shaped bodies used in a high-temperature environment, for example brake disks of vehicles, are made of the carbon-containing material. In the case of such brake disks, undesirable oxidation of the carbon fibers has in the past been observed at operating temperatures of from about 700° C. to 1000° C. As combustion or oxidation of the carbon fibers progresses, structural damage to the brake disk which has an adverse effect on its stiffness and strength may occur. In view of the expensive manufacture of such fiber-reinforced C/SiC brake components, replacement on the basis of need rather than as a precaution is preferable. However, this requires regular nondestructive testing of the brake components.
Attempts have been made in the past to determine a loss of carbon caused by oxidation by weighing components made of the abovementioned composites. However, this method is very costly since it requires removal of the C/SiC component from the assembly. Furthermore, such components were subjected to optical inspection. Although this method enables conclusions regarding the surface state of the C/SiC component to be drawn readily, it is only suitable for assessing the overall state of the component if the surface state corresponds exactly with the state of the remaining component. This is reliable enough only in very rare cases.